1. Field of the Invention
The present invention relates to a recording paper and a method for recording images according to an ink jet or electrophotographic recording process using the recording paper.
2. Description of the Related Art
The ink jet recording process has been attracting attention, as it has many advantages in that it allows easy full color printing, consumes less energy, is less noisy during recording, and utilizes printers having a lower production cost. Recently, while there is a trend toward further increase in image quality, speed, and reliability of the process, images and characters are more frequently printed on regular paper, and thus it is quite important to raise the recording compatibility with regular paper.
The mainstream of conventional ink jet printers includes those that employ a black ink that is lower in permeability into recording paper (hereinafter, sometimes referred to simply as “paper”) and uses a pigment as a colorant and color inks that are higher in permeability into the recording paper and use dyes as colorants for improvement in the quality of black characters and for prevention of bleeding among color images (inter-color bleeding).
Accordingly, especially when images higher in recording density are printed by using color inks that are higher in permeability into paper, curl and cockle generated in the recording paper immediately after printing, leading to jamming of paper and abrasion of image portions in printers. When double-sided printing is carried out on paper, it is necessary to have periods for relaxation of the curl generated in the recording paper immediately after printing and for drying the ink during double-sided printing on paper, which results in drastic decrease in printing productivity. In addition, there is also a problem in that, when images higher in recording density are printed, the curl and cockle generated after the images are left to dry become greater, whereby the requirements of high image quality and suppression of curl and cockle cannot be satisfied at the same time at a high level.
For prevention of curl and cockle after printing, there have been proposed methods of reducing curl and cockle by relaxing the stress in paper by moisturizing a freshly prepared sheet once (e.g., Japanese Patent Application Laid-Open (JP-A) No. 3-38375), by restricting the elongation in the CD of the paper in water (e.g., JP-A No. 3-38376), by restricting the ratio of the elongations in water in the MD to CD of the paper to 1.3 or less (e.g., JP-A No. 3-199081), by restricting the elongation in water in the moving direction of a portion on which ink is ejected to 2.0% or less (e.g., JP-A No. 7-276786), and by restricting the elongation in water in the CD to 1.8% or less (e.g., JP-A No. 10-46498), as well as a method of reducing cockle of a coated-type ink jet recording sheet by adjusting the amount of pigments contained in a support to 5 to 35% by weight and the internal bonding strength of the recording sheets (recording papers) to 150 to 455 g/cm (e.g., Japanese Patent No. 3172298), and the like.
Although the curl and cockle have been reported to be reduced by the methods described in JP-A Nos. 3-38375, 3-38376, 3-199081, 7-276786, and 10-46498, when ink that is rapidly permeable into recording paper is used, and the amount of the ink ejected is large, or when the printing speed is fast and the ink quantity ejected per unit of time is large, the curl becomes significantly larger and the paper is practically unusable as a document.
Alternatively, a method for reducing swelling after printing by controlling the internal bonding strength of a recording paper having an ink receiving layer to within a particular range has been described in Japanese Patent No. 3172298, but it is not sufficiently effective to prevent curl, cockle, and swelling of the paper only by adjusting the internal bonding strength. Especially when ink that is rapidly permeable into recording paper is used and the amount of the ink ejected is large, i.e., when the printing speed is fast and the ink quantity ejected per unit of time is large, the resulting paper is practically unusable as a document as it has greater cockle.
In addition, a method of reducing curl and cockle generated after paper is left to dry by controlling the irreversible shrinkage of the paper in the MD and CD caused when the relative humidity of the environment is changed to within a particular range is proposed (e.g., Japanese Patent No. 3127114). However, if the ink penetration into the recording paper is not suppressed, such a paper is not sufficiently advantageous, because when a rapidly permeable ink is used and the amount of the ink ejected is larger, the ink penetrates deep into the recording paper, increasing the absolute amount of fibers that shrink after drying overall and increasing the curl after the paper is left to dry.
Alternatively, a method of adding an ester-based nonionic surfactant having an HLB of 11 or more into an ink receiving layer has been proposed for improvement in image quality (e.g., JP-A No. 10-278409), but the ester-based nonionic surfactant having an HLB of 11 or more is too hydrophilic to cover the hydrophilic groups on a substrate (base paper) with hydrophobic groups of the surfactant, and when the ink quantity is great, the recording paper becomes more vulnerable to deformation, whereby curl becomes greater and the recording paper becomes unusable as a document.
A method of adding a surfactant having an HLB in the range of 3 to 12 to an ink receiving layer on a film surface has also been proposed for improvement in image quality (e.g., JP-A No. 62-144986), but even if the method is applied to regular paper, it is difficult to cover the hydrophilic groups of a substrate with hydrophobic groups of the surfactant since the addition amount is as low as less than 0.1% by weight, and especially when ink that is rapidly permeable into recording paper is ejected in a greater amount, i.e., when the printing speed is fast and the ink quantity ejected per unit of time is large, the paper exhibits greater cockle and is not usable as a document.
A method of size pressing using an oxidized starch obtained by a dry process has been proposed for prevention of curl (e.g., JP-A No. 2002-348798), but if this technique is used alone, elongation of the substrate due to water in the ink is great when the ink is ejected in a greater amount, and thus the resulting paper cannot be used as a document since the curl thereof is increased.
Alternatively, an ink receiving layer containing a silanol-modified polyvinyl alcohol (PVA), 11 to 20% by weight of a nonionic surfactant, and a synthetic amorphous silica as a filler has been proposed for improvement in image quality in an ink jet recording process (e.g., JP-A No. 11-115304). However, the HLB of the surfactant used is not specifically described in this patent application, and the HLB of the surfactant used in an example is 14. Thus the surfactant is too hydrophilic to cover the hydrophilic groups of a substrate with hydrophobic groups of the surfactant when it is applied to paper made from a cellulose pulp. Moreover, with this technique alone, elongation of the substrate due to water in the ink is similarly great, and thus the paper is not usable as a document since the curl thereof is increased.
Further, a method of adding a bulking softener having an HLB of 6 or less has been proposed for improvement in the bulkiness and softness of printing papers, but a surfactant having an HLB of 6 or less, and in particular an HLB of 4 or less, is not advantageous as it is less dispersible and cannot cover the hydrophilic groups in a substrate with hydrophobic groups of the surfactant, resulting in an increase in the amount of fibers that are elongated and shrunk and thus in an increase in curl (e.g., JP-A No. 2002-155494).